That is not lymphocystis that is a tumor. I wouldn't bother with meds as there is nothing that can be done. If the fish is eating and having a normal life then there isn't a concern, if the tumor continues to grow and effects the ability to eat or swim then it's time to put the fish down but until then it is what it is.
Am curious to know how you know it's not lymphocystis without doing a biopsy. I personally think it could be either a tumour or lymphcystis. Look at pictures of fish with it. Looks identical.
They don't look identical, as lymphocystis grows on the outside of skin and tumors grow from within pushing out.
I'd go for a tumor as well. It doesn't have that warty or cauliflower mass "on" the fish but looks imo to be inside the fish. That's all I can see though.
I have seen suggested meds but really no solution is known. (You might as well try salt where the fish would otherwise be put down as well as anything else).
The cause I don't really know on. Most diseases I think the aim is to cure the fish and make sure it doesn't happen again (I know obvious but sometimes seems forgotten). Here, idk, badmans suggests "oncoviruses may be suspect in turning off genes involved in tumor suppression. In fish, just as in other animals, may be benign or malignant (cancerous)."
Tropical Fish Disease Identification with pictures and cures.
The little tiny white lump thing on the top right hand corner near the fish's head. Our left obviously. It's external. If the fish were a human it would be located on the back of its right shoulder. Can you honestly not see it? That's not to say that there aren't internal cysts of the same type as the fish is a carrier of the virus. That cyst is looking after the virus. It's a virus factory. Interesting and slightly disgusting. It can attack their organs however only in extreme cases of it. Once the cyst bursts (which it will) the virus will attempt to find a new host. The virus is however host specific. But in order for it to find a host within the short period of time that it is free swimming, the new host must have a lesion or open wound on its body for the virus to enter into. What are the odds? And the new host must be a ram also. Therefore it is hardly contagious yet it still is. One day it will flatten out and maybe leave a tiny scar. But that could be months from now. Keep us posted if you want